Conventionally, pilot injection operations are performed on diesel engines to reduce combustion noise and Nox emissions. A pilot injection operation injects a minute quantity of fuel into a combustion chamber of the diesel engine prior to a main injection operation. However, for a pilot injection operation to have optimal effect (i.e., reduce combustion noise an Nox emissions), the accuracy of the quantity of the injection must be high.
The present application provides an injection quantity learning process that corrects a difference between an actual quantity of fuel injected (hereinafter referred to as the actual quantity) and a predetermined command quantity (hereinafter referred to as the command quantity) (Japanese Patent Application JP 2003-185633). According to this type of injection quantity learning process, a learning injection operation is performed when an engine is in an idle state, for example, when the command quantity of fuel for the injectors is zero. Then, the actual injection quantity is estimated based on a change of a state of operation of the engine (e.g., engine speed variation, A/F, and the like) caused by the injection. Finally, the difference between the actual quantity and the command quantity is determined and corrected. The difference between the actual quantity and the command quantity is usually caused by functional differences among the injection products including the injector.
The difference is finally corrected according to a plurality of pieces of data acquired from each of the individual cylinders. For example, N pieces of data for N cylinders are averaged to determine an appropriate correction. This is because actual injection quantities are different for different injectors even if the injectors are commanded to inject the same quantity of fuel.
However, even when utilizing the averaging method just described, old data may lead to erroneous learning or a reduction in learning accuracy. This is true because the injection products tend to function differently with age and the old data may not accurately reflect the present injection quantity difference.